Preparation of collagenous materials



0f- 13, 1964 E. KUN-rz 3,152,976

PREPARATION OF COLLAGENOUS MATERIALS Filed Sept. 2l, 1961 2 Sheets-Sheet 1 INVENTOR. 7o/sf //vrz ATTORNEY Oct. 13, 1964 E. KuNTz 3,152,976

PREPARATION OF COLLAGENOUS MATERIALS Filed Sept. 2l, 1961 2 Sheetsf-Sheet 2 "T1 ga.

INVENTOR.

Zo/55 /f/vrz TTORN EY rnnrnnarrors or United States Patent O 3,152,976 CLLAGENUS MATERHALS Eloise Kuntz, Somerville, Nul., assigner to Ethicon, Inc., a'eorporation of New lersey Filed Sept. 21, 1961, Ser. No. l-ii,5'd5 Claims. (Cl. tldmll);

`This is a continuation-in-part of application Serial No.

k305, filed January 4, i960, which in turn is a continuationtion is concerned With dye-photocatalyzed collagenous polymers and methods for their preparation.

Collagen, a proteinaceous substance present in the white connective tissue of animals, has long been employed as raw material for the preparation of leathers and surgically useful articles such as Sponges, lms, `fibers, filaments, etc. The conversion of collagen into utilitarian forms requires a tanning step wherein an agent combines with collagen, more or less reversibly, transforming the collagen into a fixed form; Tanning agents which are commonly usedin the collagen iield include formaldehyde, glyoxal, chrome `and vegetable tannins. All of the known tanning agents have one common property with respect to their tanning ability on collagenthey not only participate in the tanning reaction, but also become an integralpart of the final tanned product.

Chrome, for instance, may be a constituent of thetiinal product to the` tenure of up to 5% or more; vegetable tannin of up to 50% or more; formaldehydeof up to several percent; oil tannins of up to 10%.V Conventional tanning agents combine with various activey groups of the peptide chain as carboxyl, amino and hydroxyl groups to form bridges between lor along these chains anchored by various types of bonds, including hydrogen, covalent and salt bonds.

Among the methods currently employed for converting `liber or filament. Tanning agents are incorporated dur- ,ingV or after formation of the article.

Articles prepared according to the above-described process are based on the conversion of collagen to a dispersion wherein the collagen is in a iibrillar state 4rather than a molecular dispersion and attraction between fibrils is limited to weak and easily ruptured forces such as Van der Waals forces and hydrogen bonds. Articles prepared from such dispersions have a relatively low break strength.

Essentially, therefore, the methods heretofore employedl are predicated on the breaking of collagentissue with mild acid media and its subsequent reconstitution into newlyformed bundles of fibrils by means of alkali or dehydrating agents, usually followed by treatment for stabilization n, with agents such `as the previously mentioned tanning agents. y

It has now been discovered that novel, thermostable,

Patented ct. 13, 1064 irreversibly cross-linked collagenous polymers can be prepared by irradiating collagen-ous substances such as collagen, per se, derivatives vof collagen and collagen degradation products in the presence of a dye. ,The novel poly-V mers are distinguishable from known tannedv collagenous products in that the dye is employed .solely as a catalyst to induce ,cross-bonding reactions between amino acids of adjacent collagenchains, and, therefore, does not become chemically integrated with the final product. Thus, collagenous polymers are obtained which are structurally free from the dye which photocatalyzed their formation.

Although the mechanism of polymer formation cannot be postulated with certainty, the dye is believed 'tof photocatalyze the formation of av charge transfer complex between the photocatalyst and two amino acids of adjacent protein chains, to give a high order of cross-linking resulting in the formation of novel oollagenons polymers. The dye then extracts electrons from the amino acids, leaving ythem bound together by a covalent bond. Experiments indicate that films, filaments, fibers and Sponges made from collagenous starting materials cross-linked in this mmner possess properties characteristically different from articles formed from collagenous starting materials not so treated. Such cross-linked products have a higher tensile strength, lower water retention, less shrinkage on .heating in water at C., and less swelling in water than similar bodies formed without such treatment. Stressstrain analyses, viscosity and syneress measurements indicatethat the cross-linking is such that the collagenous polymers have increased thermostability and are much greater in size than their precursors.

In order that the nature .of my invention may be more easily understood, reference is made to the accompanying i drawings wherein;

FIGURES le through 8a are schematic representations of collagen dispersions undera phase microscope at 500V magniicationsl Referring again to the drawings, FIGURE la shows `a 0.03% unheated collagen dispersion in 0.001 N aqueous acetic acid wherein all librils are intact and randomly dispersed. FIGURE 2a shows a 0.03% collagen dispersion in 0.001 N aqueous'acetic acid following heat v treatment at 60 C. for twentyminutes with a substany FIGURE tial decrease in the number of intact fibrils. 3o: shows a 0.03% collagen dispersion in 0.001 N aqueous acetic acid followed by :heat treatment at60 C. for thirty-eight minutes with nearly complete resolution of Vthe iibrils indicating that, with time, untreated fibrils are `broken down, i.e., are not thermostable'.

l FIGURE 4a shows a 0.03 %V unheated collagen dispersion in 0.001 N aqueous acetic acid Virradiated in the presence ofv riboilavin-S-phosphate wherein the fibrils have random orientation and are well separated. Irradiation in the presence of the dye has caused linkage formation within the iibnils themselves. FIGURE 5a shows a 0.03% collagen dispersion in 0.01 N aqueous acetic acid irradiated in theV presence of riboiiavin-S-phosphate following heat treatment at 60 C. for forty minutes,gshowing that fibrils previously Virradiated inthe presence of the dye remained intact on heating, thus demonstrating the thermostability aisance o sa in the presence of riboflavin-S-phosphate followed by heat treatment at 60 C. for forty minutes showing that fibrils in the dispersion have suffered no change in orientation despite extensive hea-t treatment, indicating the thermostable character of the bonds linking them together.

Although physical-chemical properties similar to those just described may be obtained by processes of the prior art generally known as tanning the final cross-linked collagenous polymers resul-ting from the presentinvention are (in contrast to the prior art) entirelyfree from the dye photocatalyst which effected the polymerization. In this respect, the novel polymer differs from tanned collagenous products, i.e., the latter contain, as an integral part of the final structure, substantial quantities of the tanning agents used to promote'the reaction. The freedom of the novel polymers from the cross-linking agent is considered of particular importance in the light of vbiological considerations, i.e., its use in the fields of maliy cine, surgery, or for food purposes. A

According to the present invention, the conversion of native collagen to the novel polymeric thermostable products is accomplished by the exposure of these starting materials to irradation, either artificial or natural, in the presence of a dye. SinceA this fundamental discovery embraces a Wide variety of permutations and modifications, it is intended that these be included within the scope of the present invention. a

In one of its embodiments, the novel process comprises, in a general way, the preparation of a collagenous gel in which the collagenous substance, i.e., the collagen derivative, degraded collagen or, preferably, collagen is converted to afpolymerized, cross-linked form by exposing an acid dispersion of the collagenous starting material to irradiation in the presence of a photosensitive dye. The gel of polymerized collagenous material -is then shaped to the desired form, e'.g., evaporation, neutralization. or dehydration. The cross-linked gel is so bonded that articles madefrorn its possess distinctively high tensile strength and rapid in vivo absorption'time, as well as rapid in vitro digestion rate. More specifically, films and fibers. made from such gel are characterized by high `break strength, low swelling in Water, l-ow shrink temperature and rapid degradation by proteolytic enzymes. Sponges prepared from such gel are characterized by being not readily redispersible in water and being rapidly absorbable in vivo.

In accordance with the preferred embodiment of this invention, the acid dispersion is shaped and reconstituted to the desired form, i.e., by neutralization or dehydration, and the reconstituted collagenous product in situ or rewetted with water if previously dried, is subjected to the dye photocatalyst reaction'. in other words, irradiation in the presence of the dye may accompany or follow the neutralization, and complete or incomplete dehydration, and/ or evaporation.

, The term collagenous material as applied in accordance with this invention to the starting materials and to le corresponding polymeric final products is intended to embrace collagen itself, that is toy say, the protein commonly known as forming the chief constituent of connective tissue and the organic substance of bones, as well as collagen derivatives, collagen degradation products and collagen extracts.

One may therefore employ as a starting material collagen in any of its forms, i.e., native, fibrous collagen of tendon, hide, gut, fish swim bladder (ichthyocoll'); dissolved collagen in the molecular state; dispersed collagen in the fibrillar state or collagen reconstituted to the solid fibrous state from molecular or fibrillar states. in practice, the native or reconstituted collagen is immersed in, or the dispersed or dissolved collagen is added to, as the case may be, a solution of a dye and the preparation is irradiated under conditions insuring adequate light penetration, after which shaped articles are formed. Such articles .are conveniently made from the resulting polymer according to methods known to those skilled in the art and generally discussed herein.

The term collagen degradation product as used in this invention is intended to embrace partially degraded collagen, such as, for example,gelatin obtained from collagen, preferably of high molecular weight with Bloom of 205 or greater.

Collagen derivatives such as native collagen whose available side groups have been esteriiied by alcoholysis are also suitable collagenous starting materials for the preparation of the novel polymers of this invention. The certification is suitably accomplished with an alkanol having frrom about 1 to about 2() carbon atoms, preferably a lower alkanol such as methanol, ethanol, propanol, butanol, pentanol, hexanol, iso-pentanol, iso-hexanol, etc. in the presence of a dilute inorganic acid such as sulfuric,

hydrochloric, phosphoric, etc. The esterified collagen so obtained is then dispersed in aqueous solution, and subjected -to irradiation in the presence of a dye. The resulting gel is reconstituted by evaporation to the desired form. Alternatively, the esterified collagen may be directly reconstituted and the resulting product then subjected to the irradiation process.

Because the photocatalytic reaction of this invention induces a charge transfer mechanism within the collagenous starting materials, it is possible, by means of this reaction, to introduce a Wide variety of moieties into the starting materials simultaneously, in situ, during their photocatalytic transition into collagenous polymers. The charge transfer is induced by the dye photocatalyst which is capable of utilizing light energy to remove electrons from a large number of organic substances, thus creating covalent bonds suitable either for polymerization of the collagenous starting material itself, or formation of copolymers of collageneous materials with other substances or, as is usually the case, both.

An extremely Wide variety of substances may be used for participating in the charge transfer reaction of this invention, theonly limitation on the character of the material to be employed being that it should have atoms or radicals capable of donating electrons to the light activated dye. Among substances which are suitable for this purpose, are those having one or more of the following groups within their structure: amino, hydroxyl, guanidyl, amido, imido, imino,sulfhydryl, ether linkages, aldehydo, keto and functional derivatives thereof. More specifically, one may employ for this purposeamino acids such as glycine, alanine, serine, threonine, valine, leucine, isoleucine, norleucine, phenylalanine, tyrosine, cysteine, cystine, methionine, tryptophane, proline, hydroxyproline, aspartic acid, glutamic acid, histidine, arginine, lysine, hydroxylysine, citrulline, etc.; nucleic acids and nucleoproteins; carbohydrates, including monosaccharides, eg., hexoses such as glucose, fructose, galactose and mannose;

,pentoses such as arabinose, Xylose, ribose, rhamnose, and

desoxyribose; disaccharides such as maltose, lactose, sucrose, gentobiose, isomaltose and cellobiose; trisaccharides such as raiiinose; and polysaccharides such as starch,

inulin, glycogen7 dextrin, cellulose, pectin, etc.

Although it is preferred to use a dye which forms a Weak, readily reversible complex with collagenous materials (in order to facilitate easy removal, if so desired), dyes which .form relatively strong and stable complexes may be employed in cases where retention of a color in the nal product is unobjectionable or desirable. Preferred for this purpose are the compounds belonging to i.e., chromophore-containing substances capable of complexing with collagenous materials, may be employed such as those falling within, but not necessarily limited to, the following groups: tetrazoliums such as 2,3,5-triphenyltetrazolium chloride; pteridines such as 2,4( lH,3H)-pteridine d ione (lumazine); pterines such as xanthopterin; azo such as 4,4bis 7( 1-amino-8hydroXy-2,4disulfo -naphthylazo]3,3bitolyl tetrasodium salt (Evans Blue); azines such as aminodimethylaminotoluaminozine hydrochloride (neutral red); 3,7-diamino-5-phenylphenazinium chloride (phenosafranin); and tetra-ethyl phenosafranin (amethyst violet); thiazines such as 2amino-7dimethylamino-l(or 3)-methylphenazathionium chloride (toluidine blue O); 3-methylamino-7-dimethylaminophenazathionium chloride (Azure B); S-amino 7 dimethylaminophenazathionium chloride (Azure A); 2,7-diaminophenazathionium chloride (thionine) y8-nitro-2,7-bis-dimethylaminophenazathionium chloride (methylene green); 3,7-bis-(dimethylam'ino) phenazathiom'um chloride (methylene blue); indamines such as Bindschedlers Green; oxazines such as 7-oXo-phenoXazon-(2),lt}oxide (resa'zurin); acridine such as 3,6-diaminoacridinium chloride hydrochloride (proliavine); 2,7-dimethyl-3,-diaminoacridinium chloride" hydrochloride (acridine yellow); 3,6-bis-(dimethylaminoacridinium chloride hydrochloride (acridine orange); triphenylmethane such as zinc, oxalate, or ferrie double y chloride of tetramethyl para aminotriphenylcarbinol (Malachite green); liuoranes suchas 9.-(o-carboxyphenyl)f6-hydroxy- 3-isozathenone (uoroescein); the sodium or, potassium salt of tetra-iodo-iiuorescein (erythrosine); the sodium or potassium salt of tetra-bromolluorescein (eosin Y); 9-(tetrachloro-o-carboxyphenyl.f 6 hydroxy 3 isozathenone g (rose bengal); pyronines such as 3,6bis-(methylamino)r p Y.pyroninium chloride hydrochloride (acridine red 3B); and

. pH may be on the alkaline side, i.e'., from about 5.9 to

about 14.0, preferably from about 9.9 to about 11.0.

The photocatalysis proceeds smoothly with relatively t Small quantities or" catalyst as well as with larger quantities.-` Therefore, the only limitation as to the amount of dye which may be employed will be that circumscribed .by the solubility of the particular compound used. EX- perience indicates that quantities of dye ranging from about 0.2 mg. to about 50l mg. foreach 100 y grams ot dispersion are satisfactory, and that optimal results are obtained with quantities of about 50 mg. of dye for each 100 grams of dispersion. In the caseof reconstituted or native collagen one may employ from aboutv l mg.

percent to 100 mg. percent, optimally about 5 mg. percent, dye in the immersion bath.

The time requirement for ensuring completion of the reaction with a given concentration of photocatalyst varies considerably depending upon the wave length and intensity of irradiation, as well as the purpose for which the polymer is intended to be used. ln general, however, it has been found that suitable polymers are obtained with as little as two seconds exposure to irradiation and as much as several days exposure, depending upon the concentration of photocatalyst, the pH and the state of the collagenous material, as well as the intensity and wave length of light. In practice, a satisfactory period of time for polymerization to tal-:e place is from a few, eg., two or three, seconds up to several, eg., forty-eight, hours although this is not to be construed as an absolute limiting factor. Variations in light intensity and time appear unlimited except for vfactors having detrimental eifects on the collagenous material.

l and vice versa.

The most eilective wave length of light is in that range of the electromagnetic scale which is maximally absorbed by the photocatalyst, i.e., the ultraviolet and visible light regions represented in the range of from about ZSUO to about 7000 Angstrom units. Optimal results are obtained at wave lengths most strongly absorbed by th given dye. For example, in the case of isoalloxazines, optimal results are obtained at about 3600 Angstrom units in the ultraviolet range, and about 4600' Angstrom `units in the visible region. The source of visible light may be either natural, i.e., sunlight, or artificial, i.e., emanating from an arc lamp, fluorescent bulb or similar device. The process is operative at a light intensity varying from about l() foot candle power to about 13,000 foot candle power, the latter being equivalent to the intensity of sunlight. A general rule which may be followed is that the intensity employed is directly proportional to the Vtime required for the polymerization reaction to reach completion. Thus, the higher the intensity of the light, the'shorter will be the time requirement, If the operation is carried out at a relatively intensive range of foot candle power, eg., sunlight for a protracted period of time, precautions must be ,taken to keep the reaction mixture at a temperature sufliciently low to preclude denaturation of the lcollagcnous material and change in its conguration. 'Since denaturation of dispersed collagen takes place usually at about 37 C., temperatures in excess of thisvare preferably not employed with dispersed collagen, although a `temperature of about 40 C. maximum may be used in .'cases where exposure to irradiation is only for a briet period of time. if reconstituted Vor native collagen is used, temperatures below 60 C. are preferably employed.

The concentration of collagenous material in the acid dispersion or aqueous solution employed as starting material for forming reconstituted collagenous polymers may be varied over wide limits, in some cases fron1'0.0l% collagenous substance on a weight/volume basis up to about 2% to 5% but in others considerably higher. For

example, it may be desirable in some cases toprepare a relatively thick dispersion, i.e., where the polymer is intended for use in the preparation of a ilm. Under such circumstances, the operation may be carried out in media having collagenous concentration in the order oiy liminary operations under subdued light.

The preparation of the collagenous dispersion may be carried out in accordance with the methods well known to those skilled in the art. For example, the raw collagen material previously washed clean of debris, is added to an aqueous acid solution and allowed to soak until it is swollen, the swelling taking place because of the breaking ofthe hydrogen bonds` by the acid. Suitable acids which may be employed for this purpose are: sulfuric, hydrochloric, acetic, citric, lactic, etc. in a concentration from about 0.001 N to about 0.1 N. If desired, an enzyme system such as commercial maltdiastase or cin may be used to treat the collagen preliminarily to break up elastin and carbohydrate compounds.

As neutralizing agents for reconstitution there may be used organic or inorganic bases such as tertiary amines, alkali metal and alkali earth metal hydroxides, alcoholates, hydrides, arnides or hydrocarbon compounds of alkali metals or alkali earth metals. As specific examples there may be included sodium hydroxide, potassium hydroxide, pyridine and, preferably, ammonia.

Suitable dehydrating agents for reconstitution of the gel include inert water-miscible organic solvents as, for example, lower alkanols and ketones, and tertiary amines such as methanol, ethanol, propanol, butanol, pentanol, isopropanol, dioxane, pyridine, methyl-ethyl-ketone, dimethyl-ketone, diethyl-ketone, etc.

Reconstitution of the gel by evaporation is readily accomplished at room temperature, e.g., 20 C. to 30 C. Slightly higher temperatures, i.e., about 310 C. to 40 C., may be used, if caution is exercised to prevent degradation.

The novel dye-photocatalyzed polymers prepared in accordance with this invention may be converted to a variety of useful articles. For example, the polymer may be lyophilized, ie., vacuum-dried, from the frozen state to form a sponge which is not redispersible in water and which is rapidly to 25 days) absorbable in vivo. Sponges prepared in this manner from the novel polymeric gel may be used for implantation in the body for packing cavities or for hemostatis, or as matrices for tissue culture. If desired, the Sponges can be impregnated vwith drugs such as hemostatic agents, e.g., heparin; horplish this, the dispersion containing the dye is spreadout in a suitably thick layer in a dish or pan or on a screen, irradiated, and the water allowed to evaporate. Films prepared in this manner show very different characteristics from' control lrns, i.e., those prepared by conventional methods directly from collagen dispersions. Among the unique properties there may be cited: increase in break strength; decreased swelling in water; low shrink temperature; and rapid degradation by proteolytic enzymes.

The novel dye-photocatalyzed polymers may be formed at the time of reconstitution by neutralization of the dispersion or solution. To accomplish this the dispersion or solution containing the dye is exposed to the neutralizing or dehydrating agent and simultaneously irradiated. Conversely, the dye is incorporated into the neutralizing or dehydrating bath and irradiation accompanies application of the neutralizing bath. Sponges, films and fibers may be formed by this method.

The novel dye-photocatalyzed polymers may be formed from previously reconstituted collagenous material. The latter is reconstituted from an acid dispersion or a solution by neutralization and/or dehydration. The reconstituted collagenous material is then irradiated the presence of dye. Collagenous Sponges, films and fibers may be polymerized by this procedure.

It may be stated here that the dye-photocatalyzed polymers of this invention show characteristic behavior in gelation and syneresis tests which provide a basis for distinction from conventional aqueousacid collagenous dispersions. For example, when an acetic acid dispersion of collagen containing a photosensitive dye is exposed to visible light, a rapid partial setting to the gel state occurs. Continued exposure results in greater iirmness. On the other hand, control samples without photosensitive dye in the light or with photosensiti-ve dye in the dark remain in a liquid state. This simple comparison clearly establishes the necessity for both a photosensitive catalyst and light. The syneresis phenomenon is manifested. when collagenous dispersions are gradually heated. Dispersions containing no photosensitive dye undergo a marked lowering of viscosity to a watery state. Conversely, when collagenous dispersions containing a photosensitive dye are heated, the effect is reiected by the time and intensity of previous irradiation' Nonirradiated samples show liquefaction. Irradiated samples exhibit syneresis with marked separation into a gel phase, which remains set at elevated temperatures as well as at room temperature, and an aqueous phase. The relationship between the degree and permanence of syneresis andthe time or intensity of irradiation is quantitative. As used herein, the degree of syneresis is expressed in terms of the percent of liquid expelled from the contracting gel.

The following examples are intended to illustrate the invention but are not to be construed as limitative upon its scope.

Example l Aliquots of-collagen dispersion in `0.01 N acetic acid pH 4.0 are prepared (as in Example IV). Riboiiavin in aqueous solution is addedto give a iinal concentration of 0.05 mg. percent, 0.5 nig. percent, 5.0 rng. percent or 50 mg. percent riboflavin with a final collagen concentration ofV 0.3 percent, the percent change in viscosity after onehour irradiation with 400 foot candle power of daylight uorescent light is recorded.

Riboflavin concentration,

mg. percent Percent change in viscosity 0.05 3

Example II i Aliquots of a collagen vdispersion prepared as in Example IV in 0.01 N acetic acid are adjusted to pH 2.1, 3.1,

4.0 and riboiiavin is added. The final concentrations are- 0.2% collagen and 1.25 mg. percent riboavin. The viscosity changes upon irradiation at 26 C. with daylight fluorescent bulbs are recorded.

Relativo Viscosity pH 2.1

pH 3.1 pH 4.0

initial, dim iight s. 7 44. s 264. o 400 te., minutes 6. 98 850. 0 Set Example III v'formation of Sponges.

It collapses and after thirty minutes, swells and forms a r 'translucent gel which breaks upon handling. The ribosuspended in a beaker of distilled Water. just suicie'nt to keep the films straight are attached to and the separation of a thermostable gel (syneresis) is observed.

' Time of Percent Sample Irradiation, State at 70 C. Syneresis Seconds Fluid 0. 1 ..d0 0. 5 Soit flocculent gel- Slight. V Soit scattered masses Do.

of gel. 30' Single mass of gel 46. 60 .do 46.

Example IV Two grams of collagen tendon (malt-diastase treated) are dispersed in 200ml. .01 N. acetic acid. The mixture is homogenized in a Waring Blendor to give a smooth dispersion (0.5% collagen) from which are taken two 50 ml. samples. One sample is used as a control, to the other is added 1 mg. of riboilavin. Each mass is mixed in a Waring Blendor and exposed to sunlight for one hour. Both samples are lyophilized, resulting in the The control is placed in water.

avin-treated sample is placed in water. It collapses par tially, regains its original shape and dimensions, and is able to holdL forty-seven times its dry weight of water.

Example V Twenty-live grams of beef tendon slices are placed in 200 ml. of 2% malt-diastase overnight at 37 C. The slices are washed, placed in one liter Vof 0.01 N acetic acid and homogenized for730 seconds in a Waring Blendor. The thick mass is run through a Bchner funnel. To 100 acid and the mixture is hand-mixed. A control of 10ml. 0.01 N acetic acid in 100 grams of dispersion is also pre pared. l The dispersions are spread into 15 cm. Petri dishes (30 gm. per dish). One dish of riboavin-treated collagen is placed in a refrigerator, together with a control, in the dark. Two dishes of riboavin-treated collagen and two controls are placed at room temperature,

Q26" C., in room light of 25 foot candle power.

Refrigerated, D ark 26 C., Light Control Riboilavin- Control Riboflavintreated treated 3vl1ours k Fluid Fluid Fluid..." Partially set.

Vhcurs .do do -do Do. .24 hours .do- -dodo Set.

48 hours do.--. do Dried. Dried.

A strip of the 26 C. light-treated control and'a strip of 26 C. light-treated riboilavin-treated ilrn are Small weights the bottom of each iilm. The temperature of the water bath is slowly raised. The degree of shrink temperature and amount of contraction are recorded.

Original Contracted Percent Length, num. Length,mm. Contraction Control 43 18 58 Riboflavintreated 58 29 50 Example Vl To l5 grams ofa dispersion (0.5% collagen) preethanol. To another l5 grams `of a similar dispersion Agrams of the resulting dispersion (0.58% collagen) there p is added y2 mg. of riboiavin in 10 ml. of 0.01 N acetic 'paredasin Example V there is added 1.5 ml. of 50% of sixteen, the riboavin-treated gel has a relative vis? cosity of 166. V-

Example VII To grams of dispersion (0.5% collagen) prepared as in Example V there is added 2 mg. of riboflavin. Another 100 grams of similar dispersion is used as a control. The dispersions are spread in dishes and placed under light of 400 foot candle power. Another series, including control, is placed under light of 25 foot candle power: Irradiation is maintained for three days, using iiuorescent lamps as the source of light.V Water is allowed to evaporate from all dishes, the dried lm is soaked loose and tested for wet break strength on a Scott tensile strength machine.

Control, 400 ft. candle 25 it. candle lbs. lsq. in. power, lbs./sq. in. power, lbs/sq. in.

30o 2,000 Y 2, 80o 300 1, 600 1, 800 500 950 1, soo

All samples are tested for rate of breakdown when subjected tothe papain digestion time test.,

Control Completely digested in 24 hours.

25 foot exposure Completely digested in 5 hours.

400 foot exposure Completely digested in 140 seconds.

The papain digestion time test employed is as follows: A 100 mlfvolume of a buffer solution containing 174.22 grams dibasic potassium phosphate and 38.15 grams sodium borate made up to one liter of water is mixed with 76'grams thiourea, 800 ml. distilled water and 30 grams papain. Just prior to testing, a 1 ml. volume of Ian activator made by dissolving 5.2 grams sodium bisultite in water up to a 100 ml. volume is added to 24 ml. of the buffered papain solution. The film to be tested is irnmersed under twenty grams of tension in the activated and buiered papain solution, which is maintained at an incubation temperature of 37.8"y C. The period ottime required for the film to be completely digested represents papain digestion time. Y

Example VIII To 10 grams of dispersion (0.4% collagen) prepared as in Example ll there is added 1 ml. of 50% ethanol containing 0.2 mg. riboilavin. To another 1.0 grams of a similar dispersion there is added 1 ml. of 50% ethanol containing 0.2 mg. riboaVin-S-phosphate. Both samples are exposed to a 400footcandlepower fluorescent lamp for 21 .hours and the resulting gels are placed in a viscosimeter. v

Relative Viscosity Before After Irradiation Irradiation Ribonavm sa 40o Rboavin--phosphate; 335

l; l Example IX Two dispersions (0.81% collagen) of 100 grams each are prepared as in Example V, and 2 mg. riboflavin is added to each. One is exposed to ordinary room light of 100 foot candle power, the other to ultraviolet light at 3000-4000 Angstrom units. Both are irradiated for one hour, and viscosity measurements are taken at 70 C. in centistokes. Parallel controls are also run, and readings taken.

Centistolres No addition of riboflavin, room light v 0.14 Riboavin added, room light 0.74 No addition of riboavin, ultraviolet light 0.15 Riboflavin added, ultraviolet light 0.30

Example X A dispersion (0.81% collagen) is prepared and riboilavin `is added thereto as in Example IV, in a series of three aliquots and a control for each. One dispersion is irradiated under room light, the second under ultraviolet at 300D-4000 Angstrom units, the third under sunlight. All irradiations last one hour. All samples are then lyophilized to form Sponges.

Control, room light Gelatinous, disperses in water. Control, ultraviolet light Weakly iibrous. Control, sunlight Gelatinous, disperses in water.

Ribofiavin, room light `Fibrous, holds 52 times weight of water. Riboflavin, ultraviolet light Fibrous, holds 39 times weight or water. Riboiiavin, sunlight Fibrous, holds 40 times weight of water.

VYExample Xl tions `are as elective as high concentrations, but are slower,

which is expected for catalysis.

5 Minutes 20 Minutes Irradiation Irradiation mg. ribotlavin 43% syneresis 65% syneresis.

53% Syneresis. 63% Syneresis. 64% Syneresis. 58% Syneresis. 61% Syneresis.

0% Syneresis.

mg. ribotlavin 45% syneresis 1 0.6 0.4 mg. rihotlavin. 0.2 mg. riboilavin. 0.1 0.0

mg. riboavin. 5 mg. riooflavin Control Example XII A dispersion is prepared (0.81% collagen) and riboflavin added thereto as in Example IV. The dispersion is irradiated under room light at 25 Ifoot candle power for.

gion of a series of rats. The animals are autopsied at intervals and time for absorption `and disappearance of the sponge is observed.

Time for Tissue Absorption, Reaction Days Rboavin-treated 15-25 1 Formaldehyde-treated 40-80 4-7 Tissue Reaction; 0-none, 2-slight, Li--rnoderate, G-rnarked, 8-severo Example XIII Three grams of collagen tendon (malt-diastase tre-ated,

washed and lyophilized) are placed in 300 ml. of absolute methanol containing 0.036 N sulfuric acid. The product is stored at 2 C. for ele-ven days with occasional shaking. The material is Washed thoroughly with methanol and dialyzed against distilled water for two weeks at 2 C. and dried with acetone. The product, methylated collagen, readily disperses in water.

Six live-grams samples of a 5% dispersion of esterilied collagen dissolved in water are prepared, one sample serv- `ing as a control. To each of three samples (controls) is added a 0.5 ml. volume of water. To each of the remaining three samples a 0.5 ml. volume of water containing .25 mg. of riboavin is added. The samples are irradiated under 400 foot candles ot daylight uorescent light, heated `at 70 C. for twenty minutes, the degree of Syneresis is Example XIV y A dispersion containing 0.3% collagen is prepared by treating tendon slices with 0.02%v licin, inactivating the iicin with 0.1% hydrogen peroxide and swelling the slices in 0.01 M acetic acid, clarifying by centrifuga-tion, precipitating in Iacetone yand redispersing to for-m a 0.9% dispersion in 0.1 M acetic acid. The dispersion is extruded through a hypodermic syringe into acetone. The resulting libers are dried and divided into two lots. One, serving as a control, is placed in phosphate buffer, pH 9.0. The second lot is placed in phosphate buler pH 9.0 containing 50 mg. percent riboliavin. The two preparations are irradiated for sixteen-hours at-200 foot candle power.

The degree of cross-linking is determined by stress-strain analysis las adapted to collagen by Wiederhorn et al., I. Am. Leather Chem-ists, Vol. 48, pp. 7-20, 1953. This technique determines the molecular weight between crosslinks of polymeric materials. The lower the molecular weight between cross-links, the greater the degree of crosslinking. The results given below are expressed in terms of molecular weight between crosslinks.

Average Molecular Number of Weight Cross-links/ Between Tropocollagen Cross-links Unit of 360,000 inw.

Control 151, 000 2. 4 Riboflavin-treated 21, 000 17. l

In like manner, fibers prepared by theprocedure described above are subjected to treatment under the same Acridine yellow.

VNeutral red Erythrosine conditions, including controls, except that other dyes are l substituted for the riboiiavin. The results are given below.

Averagev Number of Cross-links] Tropocollagen Unit of 360,000 1n.w.

Molecular Weight Between Cross-links i Concentration Dye (mg. percent) Xanthopterin 2,3,5-triphenyltetrarolium Cl- Acridme red.

Bindscliedlers green- Primuline Fluorescein Malachite green Example X V .f rng.l of riboiiavin dissolved in 60 ml. of water into 600 of dispersion. The dispersion is spread on a screen,

"neutralized by spraying with ammonium hydroxide, and

then irradiated with 200 foot candles during drying. A third ilm is prepared by adding 5l mg. ribollavin dissolved in 108. ml. water to 1,080 gm. of dispersion.. The

dispersion is spread on a screen and irradiated for Vsixteen hours with 200 foot candles of light and thenneutralized with a spray of ammonium hydroxide Vand allowed'to dry. `The molecular weights between cross-links are given Control k1,191,000 Riboiiavin-neutralized-then irradiated 22,200 Riboiiavin-irradiated-then neutralized 50,600

Three pieces, 2" by 5 in size of the control` film (supra) are subjected to irradiation in riboiiavinsolutions (5,0 mg; percent), and buffered with 0.1 M phosphate at pH 5.0, 7.0 and 9.0, under 200 foot candle power for a period yof twenty-three hours. The degree of cross-linking is determined by stress-strain analysis.

Molecular Weight Cross-links] Between 360,000 Cross-links v Example XVI Twenty-live grams ofl .tendon slices in 250 ml. of a`1`% saline solution are agitated in the cold (4 C.) for twenty- Vso . solution sets to a iirm gel within one minute.

fours hours. The solution is separated by decantation and a suliicient quantity of sodium chloride is added with stirring to bring the solution to 23% salt concentration. Stirring is continued until all of the salt goes into solution, and the solution is allowed to stand at 4 C. for about twelve hours. A gel-like precipitate of collagen accumulates at the surface which is separated from the clear liquid by centrifugation. Itis then dissolved in sufficient 1% saline to make a 0.5% concentration.

To two grams of the above solution are added 0.5 ml. ot distilled water as a control. To another 2 grams of the above solution are added 0.5 ml. of distilled water, con- V taining 0.06 mg. riboiiavin. Both preparations areA placed in sunlight of 800 foot candle power. The control remains iiuid for thirty minutes. The riboavin-containing Both are heated to 70 C. for twenty minutes. The control remains uid whereas the riboflavin preparation synereses into a liquid phase and gel phase with 38% syneresis.

Example X VII Commercial gelatin is spray-dried by conventional means to form a finely divided powder. To a l0 m1. volume of a 2% ysolution of this gelatin in warm water is added a 0.5 mg. quantity of riboilavin dissolved in 1 m1. water. One ml. of water is added to a control sample. The two samples are irradiated for sixteenhours with 400 foot candles of light and then heated to 70 C. The control sample liqueies. The riboilavin-treated sample `remains set.

' VExample XVIII 0.5 gm. iilms of purified pigskin gelatin with an isoelectric p-oint of 8.92 are placed in 10 ml. of 0.1 M VNazi-IPO., butter pH 8.6 containing l5.0 mg. of riboilavin- 5phosphate. Samples of each are irradiated with 400 foot candle power light from daylight fluorescent bulbs for sixteen hours. Control samples are kept in the dark. All films are then placed in water and heated to C. for sixty minutes.

Bath Treatment Eiect of Heating to 100 C.. for 60 minutes Buier- Dark. Melted. Do Do. Buicr Ribollar n D lr Do. p Do Film intact and iirm.

Example XIX Films of puriiied pigskin gelatin with an isoelectric fpoint of 8.92 are placed in 0.1 M NagHPO., buffer containing 50 mg. percent rboflaVin-S-phosphate and irradiated at 400 foot candle power with daylightliuorescent bulbs for sixty minutes, thenrernoved from the solution and air-dried at room temperature under continued irradiation. The molecular weight between thermostable crosslinks is determined by stress-strain `analysis* at 92 C.

The average molecular weight between cross-links in the polymer is 162,000. Control films melt.

Example XX 30 times its weight for l0 seconds drain time. 28 times its weight for 30 seconds drain time. 24.5 times its weight for 60seconds drain time.

l Example XXI ameter averaging about 100 microns long. When heated to -'S6-61" C. the brils of the control preparation slowly become less visible and break up intoy bead-like segments before finally disappearing. The riboflavin-S-phosphatetreated fibrils remain intact and unchanged in appearance. (See FGURES 1 to 5.)

Example XXII To grams of collagen dispersion, prepared as in Example XV andy containing 0.6% collagen in 0.01 N acetic acid, is added 5 mg. of riboilavin-S-phosphate dissolved in 5 ml. of Water. The dispersion is irradiated with 200 f.c. of light from daylight fluorescent bulbs for two hours. The dispersion rapidly sets to a firm gel. The gel is broken up into small pieces in water, examined and photographed under a Zeiss phase contrast microscope. Most of the fibrils are fastened together in bundles of closely packed parallel fibrils (see FIGURE 4). The fibrils do not separate from the bundles under moderate stress. Under more severe stresses the bundles would shear into smaller bundles of fibrils.l When heated to 56--61'o C. for forty minutes, the bundles of fibrils rcmain intact and unchanged in appearance. The control dispersion contains separate, randomly oriented fibrils. These slowly disintegrate upon heating to 5661 C. (See FIGURES 6 and 8).

Example XXIII in one' ml. of a solution containing 50 mg. percent riboiiavin-S-phosphate in 0.1 N disodium phosphate plus the indicated radioactive amino acid. The samples are irradiatedwith 200v foot candles of light from daylight fluorescent .bulbs for sixteen hours. Control samples are kept in the dark for the same period of time. are then washed in twenty changes of cold water plus three changes of 70 C. water to remove unbound amino acid. The samples are dissolved in one ml. of hyamine hydroxide solution and 10ml. of toluene containing'0.3% 2,5-diphenyloxazole and 0.1% p1-bis-2(5-phenyloxazolyl)benzene as scintillators. Carbon14 is measured in a scintillation counter. Results are shown below.

The samples Percent of Added l/igllicoof Amino Acid Bound' Amino Acid Acid by Collage Added Dark Illuminated Tyrosine 2 C14 0.41 3. 4 71. 0 L-Methionine, Methyl C14.. 0.17 5. 5 44. 4 DL Methionine, 2 C14. 1. 93 0.3 7. 2 Glycine 2 C14 y 0.13 6.3 31.2 Glycine 1 C14..-" 0. l1 2. 0 1l. 5 DL Hlstidine 2 C14 0. 96 1. 6 l0. 5 L Aspartic Acid, 2-3 C14 0.25 1.6 6.5 DL Alanine, 1 C14 0. 6 0. 2 1`. 0

What is claimed is:

1. A method for preparing an irreversibly cross-linked', thermostable collagenous polymer which comprises the step of irradiating a collagenousv starting materialv inthe presence of a photosensitive dye photocatalyst.

2. A method as set forth4 in claim 1 wherein, the starting material is in the form of an acid dispersion.

3. A method as set forth in claim 1 wherein the starting material. is. in the. form .of an aqueous.Y Solution.Y

4. A method as set forth in claim 1 wherein the. starting material isesteried collagen.

5. A method as set forth in claim 1 wherein the. starting material is a collagen derivative.

' 6. A method as set forth in claim 1 wherein the starting material, is a collagenderivative formed in situ during the photocatalytic reaction by photocatalysis.

7. A method as set forth in claim l wherein the starting material is native collagen.

8. A method as set forth inclaim 1 whereinl the starting material is partially degraded collagen.

9. A method as set forth in claim l wherein the starting material is` gelatin.

10. A method as set forth in claim 1 wherein the starting material is reconstituted collagen.

l1. A method as set forth in claim 1 wherein the starting material is reconstituted collagen in the form of a film.

12. A method as set forth in claim 1 wherein the starting material is reconstituted collagen in the formof a liber.

13.. A method as set forth in claim 1 wherein the starting material is reconstituted collagen in the form of a sponge.

' 14. A method as set forth in claim 1 wherein the starting material is reconstituted collagen in the form of a lament.

15.` A method as set forth in claim 4v wherein the esteriied collagen is dispersedin water.

16. A method asset forth in claim. 1 wherein 'the irradiation is carried out under natural light.

17. A method as set forth in-clai'm l wherein the irradiation is. carried out under artificial light.

18. A method as set forth in claim 1i wherein. the irradiation is carried out at a wave length of from about 2500 to. about 6000 Angstrom units.Y

19. A method' as set forth` in claim l wherein the irradiation is carried out at an intensity of from about 10 to about- 13,000 foot candle power.

20. A method asV set forthv in claim 1 wherein the catalyst is photosensitive 6,7-(R,R1)9(l-X) isoalloxazine, R and R1 being, interchangeably, members selected from the group consisting of hydrogen atomsand lower alkyl radicals, and X is a member selected from the group consisting of ribityl, phosphoribityl, monoacetoribityl, polyacetoribityl', malonyl, the water-soluble sal-ts and dinucleotide derivatives thereof.

References yCited in the file of this patent UNITED STATES PATENTS 2,494,041 Frame Jan., 10, 1950 2,507,244 Correll May 9, 1950 2,850,445 Oster Sept. 2, 1958 2,923,673 Munger Feb. 2, 1960 2,948,706 Schellenberg et al. Aug. 9, 1960 OTHER REFERENCES Oster: Photographic Engineering, volume 4, No. 3 (1953), pages ,176 and 177.

UNITED STATES PATENT oE-EICE CERTIFICATE OF CORRECTION Patent No., 3, 152,976 October. 13, 1964 Elois Kuntz It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 30, for "rradation" read irradiation column 4, line 19, for "certification" read esterification column 5, line 37, for "7-oxo-"' read 'k-oxy- Signed and sealed this 16th day of February 1965,.

(SEAL) Attest:

EDWARD J. BRENNER ERNEST W. lSWIDERV CommissionerV of Patents Attesting Officer 

1. A METHOD FOR PREPARING AN IRREVERSIBLY CROSS-LINKED, THERMOSTABLE COLLAGENOUS POLYMER WHICH COMRPISES THE STEP OF IRRADIATING A COLLAGENOUS STARTING MATERIAL IN THE PRESENCE OF A PHOTOSENSITIVE DYE PHOTOCATALYST. 